The structure of a related art rotational angle detecting device will be briefly described with reference to FIGS. 27A–27B. The rotational angle detecting device comprises a rotor J2 formed of magnetic material which has magnet (hereinafter referred to as “main magnet J1”) divided in a radial direction so as to form a substantially cylindrical shape with a magnetic pole facing in one direction, and a magnetic detection element J3 (for example, Hall IC) fixedly disposed in the rotor J2. The main magnet J1 has magnetic poles confronting in the same direction. When the relative rotational angle between the main magnet J1 and the magnetic detection element J3 is varied, the magnetic flux density (magnetic force) passing through the magnetic detection element J3 is varied, so that the output signal of the magnetic detection element J3 is varied. That is, the rotational angle detecting device detects the relative rotational angle of the member at the main magnet J1 side and the member at the magnetic detection element J3 side on the basis of the output signal of the magnetic detection element J3.
FIG. 2A shows the typical relationship between the rotational angle and the magnetic flux density passing through the magnetic detection element J3 (the magnetic flux density passing through the magnetic detection element J3 will be hereinafter merely referred to as “magnetic flux density”) in the rotational angle detecting device having the above structure. It is assumed that the output range of the magnetic detection element J3 in connection with the magnetic flux density variation when the main magnet J1 is clockwise rotated around the magnetic detection element J3 in FIG. 27A is set to a positive side while the output range of the magnetic detection element J3 in connection with the magnetic flux density variation when the main magnet J1 is counterclockwise rotated around the magnetic detection element J3 in FIG. 27A is set to a negative side. In this case, when the use output range of the magnetic detection element J3 is used at any one of the positive and negative sides, the magnetic density is turned back at ±90 degrees, and thus the detection limit is in the range of ±90 degrees.
In general, a permanent magnet is used as the main magnet J1. The permanent magnet has a temperature characteristic that the magnetic flux varies with the temperature. However, the variation of the temperature characteristic is a minimum when the magnetic flux density is equal to 0 (mT, the unit is hereinafter omitted). Therefore, the magnetic detection precision at the magnetic flux density around 0 is enhanced.
Here, when the rotational angle detecting device is applied to a means for detecting the opening degree of a throttle valve, it is required to detect a minute opening degree around the opening degree under idling with high precision, and thus the magnetic density around 0 is required to be used as the 0° position of the throttle valve.
In this case, the detection range of the throttle valve is limited to the range from 0° to 90°, and thus an opening degree of 90° or more cannot be detected.
That is, the detection of 90° or more is impossible not only in the case where the conventional rotational angle detecting device is applied to the throttle vale, but also in the case where the reference angle of the angle 0° is set around the magnetic flux density of 0, and thus the detection angle range is limited to the range from 0° to 90°.
Furthermore, in order to enhance the detection precision, it is required that the variation characteristic of the magnetic flux density in the detection angle range has linearity (hereinafter referred to as “linearity”). However, the output of the magnetic detection element J3 has a sine-curve variation, so that the detection range having high linearity is narrow and the linearity would be lowered as shown in FIG. 2A if the detection range is broadened.
As a means of solving the above disadvantage has been proposed a technique, as shown in FIGS. 28A–28B, that an auxiliary magnet J4 is disposed in the neighborhood of the magnetic detection element J3, and the detection angle range is enlarged to 90° or more or the detection range having high linearity is broadened by applying constant magnetic force in the magnetic detection direction of the magnetic detection element J3 while the reference angle of the angle 0° is set in the neighborhood of the magnetic flux density of 0 at which the magnetic flux density is not varied by the temperature characteristic (non prior art: for example, JP Application 2002-285378).
In the technique disclosed in JP Application 2002-285378, as shown at the lower side of FIG. 29, when the auxiliary magnet J4 is merely disposed in the neighborhood of the magnetic detection element J3 to apply constant magnetic force in the magnetic detection direction of the magnetic detection element J3, variation of the distance between the auxiliary magnet J4 and the magnetic detection element J3 (represented by the distance in the X-axis direction in FIG. 29) greatly varies the magnetic flux density detected by the magnetic detection element J3 as shown in the graph at the upper side of FIG. 29 because the auxiliary magnet J4 forms an open magnetic path. Accordingly, the rotational angle of the main magnet J1 for the magnetic flux density of 0 is varied after the offset based on the auxiliary magnet J4, so that the detection precision is lowered.
Furthermore, in the case of use of a stator yoke J5 which is not varied in relative rotational angle with respect to the magnetic detection element J3 and effectively leads magnetic flux occurring in the main magnet J1 to the magnetic detection element J3 as shown in FIG. 30, the auxiliary magnet J4 forms a closed magnetic path, and thus the effect of the displacement of the relative distance between the magnetic detection element J3 and the auxiliary magnet 14 can be suppressed to a small level. However, the use of the stator yoke J5 increases the number of parts. Furthermore, hysteresis occurs in the magnetic flux density applied to the magnetic detection element J3 due to the residual magnetic flux of the stator yoke J5, so that the detection precision is deteriorated.